1. Field of the Invention
This invention pertains to the field of aquaculture and in particular to the hydroponic cultivation of plants.
2. Description of the Related Art
The integration of aquaculture and the hydroponic cultivation of plants has been examined repeatedly over the past three decades with a wide variety of system designs, plant and aquatic animal species, and experimental protocols (Rakocy and Hargreaves 1993). This research was stimulated by the perceived merits of hydroponics as a method of reducing pollution from fish farms by removing nutrients from the culture water while generating additional revenue through the sale of hydroponic plant products. Closed, recirculating systems have generally been identified as the most appropriate aquaculture system types for integration with hydroponics (hereafter referred to as "integrated systems") because nutritional and environmental conditions can be maintained at levels sufficient for hydroponic plant culture (Nair et al. 1985). Despite the superiority of recirculating systems over other aquaculture system types, the prevalence of nutritional deficiencies and yield reduction in plants in prior studies utilizing recirculating systems, caused by deficient nutrient solutions and/or excessive salt accumulation, indicated that optimal nutrient concentrations could not be maintained over prolonged periods of time if commercially-available diets were used (Van Toever and MacKay, 1981; Sutton and Lewis, 1982; Burgoon and Baum, 1984; Wren, 1984; Fitzsimmons, 1985a, 1985b, 1985c; Nair et al., 1985; Zweig, 1986; Rakocy and Nair, 1987; Rakocy, 1989a, 1989b; Rakocy et al., 1989; Rakocy and Nair, 1987; Rakocy, 1989a, 1989b; Rakocy et al., 1989; Parker et al., 1990; Clarkson and Lane, 1991; Rakocy et al., 1993). As a result, continuous monitoring of nutrient concentrations, nutrient supplementation, and/or water replacement were required to correct for nutrient deficiencies and salt accumulation.
For a given integrated system operating at steady state with no nutrient supplementation, nutrient concentrations will theoretically increase, decrease, or remain constant over time if nutrient production by the fish is greater than, less than, or equal to the nutrient assimilation by plants and other losses, respectively. The rate of change in nutrient concentration can be influenced by varying the ratios of plants to fish (Rakocy et al. 1989; 1993), but the rates of change in concentration for individual nutrients differ because the relative proportions of soluble nutrients excreted by fish do not mirror the proportions of nutrients assimilated by plants. The disparity in accumulation (or reduction) rates of different nutrients quickly results in suboptimal concentrations and ratios of nutrients, thereby reducing the nutritional adequacy of the solution for plants. Accordingly, there does not appear to exist an optimal ratio of plants to fish capable of sustaining nutritionally-adequate nutrient solutions if standard fish diets are used.
The development of fish diets having modified mineral content (hereinafter referred to as "designer diets") has been suggested as a possible means of influencing the accumulation rates of nutrients and reducing or obviating the need to supplement nutrients artificially (Seawright, 1993). Theoretically, the mineral content of a diet can be manipulated to make the relative proportions of nutrients excreted by fish similar to the relative proportions of nutrients assimilated by plants. With such a diet, there would theoretically exist an optimal ratio of fish to plants resulting in optimal nutrient concentrations through time without substantial nutrient supplementation. According to Liebig's law of the minimum, the nutritive constituent present in a limiting amount determines the yield of a plant crop, provided that other environmental factors controlling growth are non-limiting (Douglas 1985). Accordingly, the low concentrations of several important plant nutrients in fish excreta limit plant growth in integrated systems if not supplemented to the solution. If the concentrations of these nutrients in the excreta were increased by augmenting the diet, potentially greater yields and greater utilization of previously underutilized nutrients might be realized.
The predominant technique used in the commercial hydroponics industry to maintain appropriate nutrient concentrations is to begin with a complete nutrient solution and partially or fully replace it on a periodic basis (Resh 1989). During the interim, changes in nutrient concentration are indirectly monitored by measuring the electrical conductivity of the solution and pH and nutrient concentration adjustments are made by metering in acids, bases, or nutrient solutions. The replacement of the nutrient solution is typically justifiable for commercial hydroponics growers because fertilizer comprises only a small component of the operating costs, while the complete replacement of nutrient solutions in integrated systems defies a central rationale for integrating aquaculture and hydroponics, namely the reclamation of otherwise wasted nutrients. In integrated systems, nutrients are continuously being lost in water discharged to help maintain water quality and remove solids. Therefore, if a dynamic balance between nutrient assimilation by plants and nutrient addition is established, periodic solution replacement becomes unnecessary.
Maintaining appropriate nutrient concentrations for plants is crucial because nutrient deficiencies, which dramatically lower yield and market appeal, quickly ensue. Once nutrient-specific deficiencies appear in plants, dramatic, often non-reversible physiological changes have already occurred. Hence, the management of the nutrient profile by the addition of exogenous nutrients once clinical deficiency signs appear is an impractical approach. Alternatively, assays for periodically tracking nutrient profiles, such as atomic absorption spectrophotometry or inductively coupled plasma emission spectrophotometry, are practical only for research facilities and the largest commercial hydroponics operations because of high costs and the technical expertise required. Older colorimetric and titrimetric techniques, while reliable and technically sound, are time consuming and require adequate facilities and expertise. The development of designer fish diets, which would maintain nutrient profiles at near-optimum levels for prolonged periods by creating a dynamic balance between nutrient inputs and outputs, may therefore be of commercial value.
The creation of a designer diet development protocol requires extensive quantitative information on the flow of nutrients through integrated systems. But because the possible combinations of plant and aquatic animal species, system components, operational protocols, and environmental parameters are virtually limitless, a practical approach is to first study nutrient flow within a representative integrated system. Using data derived from a representative integrated system, a dietary inclusion model might be developed, based on principles conserved among all integrated systems.
The concept of designer diet development relies fundamentally on the principle that excretion of soluble minerals by fish will increase in response to increased concentrations or quantities of minerals in the diet. Accordingly, the initial objective of this study was to determine the effect of differing fish biomasses (and hence feed mineral input) on the accumulation rates of different minerals within culture systems having equivalent plant yields. This would serve as a simulation of differing dietary mineral concentrations for those minerals for which soluble nutrient excretion increases with dietary concentrations. Positive responses would indicate which minerals are likely candidates for dietary manipulation. Differences in individual mineral accumulation rates would support the general finding that minerals accumulate at different rates when standard fish diets are used and would indicate that creating equilibrium concentrations for numerous minerals would require differential mineral supplementation.
The ultimate objective of the research was to use quantitative data derived during initial experiments to develop and adequately test a model that would predict the dietary inclusion rates of minerals necessary for maintaining static dissolved mineral concentrations at near-optimal levels for hydroponically-grown plants. This would be accomplished by comparing the changes in concentrations of dissolved minerals between treatments receiving a designer diet and those receiving a control diet not having mineral supplementation above and beyond the requirements of the fish. In the designer diet, the candidate minerals would be manipulated according to the dietary inclusion model. Obtaining equilibrium mineral concentrations at the predicted dietary mineral concentrations would verify the model.